The present invention relates to a process for the preparation of random copolymers of butadiene and isoprene and to such copolymers obtained by this process.
When preparing random copolymers of butadiene and isoprene, i.e. copolymers which are supposed to exhibit an equiprobable or chance distribution of butadiene and isoprene units, it is known to use catalytic systems in particular comprising a salt of a rare earth metal suspended in an inert hydrocarbon solvent, an alkylating agent and a halogen donor.
Butadiene in fact polymerises substantially more rapidly than isoprene with such catalytic systems, resulting in a succession of butadiene units at one end of the copolymer chain of the resultant copolymers, and a succession of isoprene units at the other end.
In its examples of embodiment, European patent specification EP-A-629,640 teaches, for the preparation of random copolymers of butadiene and isoprene, the use of catalytic systems based on:                diisobutylaluminum hydride as alkylating agent, mixed with n-butanol or triphenylsilanol and butadiene,        neodymium neodecanoate or praseodymium octanoate, as rare earth metal salt, and        allyl bromide, as halogen donor.        
One major drawback of these catalytic systems lies in the variability of the glass transition temperatures (Tg) of the copolymers obtained when using analogous catalytic systems and polymerisation conditions (see Examples 3 and 4 in which the value of Tg are −97° C. and −90° C. respectively).
Another drawback lies in the fact that each copolymer obtained exhibits a butadiene:isoprene unit content (varying from 1.35:1 to 1.7:1) which differs substantially from the butadiene:isoprene monomer content (approx. 1:1) used for the polymerisation, which means that the butadiene and isoprene are not equally reactive during polymerisation.
As a result, the catalytic systems described in this document do not permit satisfactorily reproducible production of copolymers which are truly random, i.e. in which the butadiene and isoprene exhibit an equiprobable distribution.